Quote:
I tried to do the second question via combinatorics, but i am not able to figure it out, please check the below method and guide where i went wrong
= all three in one box +2 in one box and the last one in a different box + all three in different boxes
= 3c3*7+3c2*7c1*6c5+3c1*7c3
= 7+ 3*7*6+3*7*6*5
= 7 + 126 + 270 = wrong
Think it in this way,
First letter can go to any 7 post offices
Same case with second and same case with the third letter as well so 7*7*7